We propose a comparative study of different heme proteins exhibiting oxygenase, oxidase or peroxidase activity. By characterizing the electronic structure of the heme active site, in particular of the central iron, in all available states we hope to elucidate the factors controlling the diverse reactivities of these proteins. Our main probe, Mossbauer spectroscopy, is sensitive to the heme iron only and reveals its interactions with the immediate ligands and with nearby paramagnetic centers. Complementary EPR studies are carried out when ever possible. Specifically, the following systems will be investigated: (1) Cytochrome P450 from the camphor hydroxylase of Pseudomonas putida. This system will be studied in several reaction intermediates, and single crystal EPR experiments are planned. (2) Nitrite reductase/cytochrome oxidase and cytochrome c551 from Pseudomonas aeruginosa. Here the possible interactions between heme c and d of the oxidase and between the oxidase and cytochrome c551 are of prime interest, and it is planned to study them with differentially enriched proteins. (3) We will extend our analysis of the higher charge states, compounds I and II, of horseradish peroxidase to related compounds.